Monkey puzzle

Among scholars of the game of rock-paper-scissors, only a tiny minority also study monkeys. This fact, by itself, may explain why no studies were published until 2005 about what happens when monkeys play rock-paper-scissors.

Daeyeol Lee, Benjamin P McGreevy and Dominic J Barraclough of the University of Rochester, New York, wrote the first, and so far the only, report on the subject. "Learning and Decision Making in Monkeys During a Rock-Paper-Scissors Game" was published in the journal Cognitive Brain Research. The test subjects were male rhesus monkeys. No one explained the rules: rock breaks scissors, scissors cut paper, paper covers rock. The scientists wanted to see whether the monkeys would learn from playing game after game after game. Each monkey got a sweet drop of juice after every tie, two drops after a win. It received nothing after a loss.

The scientists did not use real rocks, paper or scissors. Instead, they rigged a computer to display crude patterns of dots and circles. Different patterns represented a rock, a sheet of paper and a pair of scissors. The monkeys were not informed which symbol stood for what object. Each monkey sat in a chair, facing a monitor that flashed the symbols for rock, paper and scissors. Rather than being asked to make the traditional gesture for rock, paper or scissors, the monkey was expected to cast its gaze towards the symbol of its choice. The scientists tracked each monkey's eye movements and recorded the entire sequence of its choices.

There were only two monkeys. They worked hard. One monkey played the game for 41 days, making a total of 87,200 choices, an average of 2,127 rounds a day. The other played for 52 days, making a total of 82,661 choices, an average of 1,589 rounds a day. Over the long haul, each monkey chose paper about as often as it picked scissors. Both displayed an aversion to rocks.

The scientists used economics theory to critique the performance, saying: "Each animal displayed an idiosyncratic pattern substantially deviating from Nash equilibrium." This is a concept formulated by John Forbes Nash, awarded a Nobel prize in 1994 for his "pioneering analysis of equilibria in the theory of non-cooperative games". He was the subject of the 2001 film A Beautiful Mind. Since only two animals were tested, the team admitted that it was difficult to conclude exactly which strategies a monkey used. "This remains to be investigated in future studies."

· Marc Abrahams is editor of the bimonthly magazine Annals of Improbable Research and organiser of the Ig Nobel Prize